Gypsum board

ABSTRACT

Described herein is a composite board and a method of producing a composite board, the board having increased fire endurance. The board comprises a sheet having a thickness greater than about 0.014 inches, and a thermal conductivity of about 0.1 w/(m.k.) or less. The composite board can be part of a wall assembly comprising two boards defining an interior cavity, the sheet facing the interior cavity.

BACKGROUND OF THE INVENTION

Gypsum products can be manufactured using a slurry formed from at leastwater and stucco. The stucco, which is calcium sulfate hemihydrate(CaSO₄.½H₂O), reacts with water to form gypsum, which is calcium sulfatedihydrate (CaSO₄.2H₂O). The two water molecules in crystallized gypsumare chemically bound to the calcium sulfate in what is often termed“crystallized water.”

Gypsum wallboard is generally a composite board comprising a core 104,face sheet/liner 106, and back sheet/liner 108 (FIG. 1). In a wallboardassembly, the face sheet 114 of the wallboard is exposed to theexterior, and the back sheet 116 is placed inside a cavity defined bytwo wallboards (FIG. 2). Gypsum wallboards are commonly used in drywallconstruction of interior walls and ceilings, and should be able towithstand both fire and excessive temperatures. As a result, gypsumwallboards are manufactured using specifications that maximize fireendurance/resistance.

Fire endurance/resistance of gypsum wallboard is measured by the periodfor which a board can withstand a standard fire test. The fireresistance of a wallboard is classified according to the ability for awallboard to avoid an increase in temperature, flame passage, andstructural collapse. In order to have various parties, includingconstructors, occupants, and regulating bodies, evaluate the fireendurance on a common basis, fire test assemblies are categorized intoseveral standard arrangements. Some common assemblies include testdesigns defined by Underwriters Laboratories, Inc. (UL®), a testing andcertification agency, which has tests that are referred to as U305,U419, and U423.

A standard fire test is customarily conducted in accordance with therequirements of ASTM E119. In such tests, a fire resistanceclassification can be established based on the time at which a wallassembly shows excessive temperature rise, or passage of flame, orstructural collapse. Failure of the test occurs when the averagetemperature as measured by several thermocouples on the unexposedsurface increases more than 250° F. above ambient temperature, or anyindividual thermocouple rises more than 325° F. above ambienttemperature. The duration of fire endurance of a system is not onlydependent upon the gypsum board used in the system, but also dependsupon many other factors, including wall assembly thickness, stud typeand spacing, board size, insulation type, and others.

Although existing techniques are useful in extending wallboard fireendurance and resistance, further improvement is always desirable.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention provides a composite gypsum boardcomprising a core comprising set gypsum formed from at least water andstucco. The gypsum core has a substantially flat shape at a generallyuniform thickness, and defines the first and second board faces inopposing relation. The gypsum core comprises a sheet disposed in bondingrelation along the first board surface. The sheet is made of a materialthat has a thickness greater than about 0.014 inches, possesses athermal conductivity that is less than about 0.1 w/(m.k.), and isapplied to the back side of the finished board. The board has a dryweight of less than about 2000 lbs/1000 ft² when at a thickness of about% inch.

In another aspect, the present invention provides a method for making acomposite gypsum board comprising a gypsum core comprising forming amixture of at least stucco and water to make stucco slurry, disposingthe stucco slurry between two cover sheets and forming a flat,relatively uniform layer to create a board perform, cutting thecontinuous board preform into a board of predetermined dimensions afterthe slurry has hardened sufficiently for cutting, and drying the board.At least one cover sheet (e.g., back cover sheet) has a thicknessgreater than about 0.014 inches, and a thermal conductivity lower thanabout 0.1 w/(m.k.). The board has a dry weight of less than about 2000lbs/1000 ft² when at a thickness of about % inch.

In another aspect, the present invention provides a composite gypsumboard which comprises a set gypsum core disposed between first andsecond cover sheets. The second cover sheet (e.g., back cover sheet) hasa thickness greater than about 0.014 inches, and a thermal conductivityof about 0.1 w/(m.k.) or less. The board has a dry weight of less thanabout 2000 lbs/1000 ft² when at a thickness of about ⅝inch. When theboard is disposed in a fire endurance index test apparatus and thesecond cover sheet (e.g., back cover sheet) faces the door of thetesting apparatus, the Fire Endurance Index (FEI) of the board isgreater than about 50 minutes.

In another aspect, the composite gypsum board is produced by a methodcomprising forming a mixture of at least stucco and water to make aslurry, disposing the slurry between two cover sheets to form a boardpreform, cutting the board preform into a board of predetermineddimensions after the slurry has hardened sufficiently for cutting, anddrying the board. At least one cover sheet (e.g., back cover sheet) hasa thickness greater than about 0.014 inches and a thermal conductivityof about 0.1 w/(m.k.) or less. The board has a dry weight of less thanabout 2000 lbs/1000 ft² when at a thickness of about ⅝inch. When theboard is disposed in a fire endurance index test apparatus and the coversheet that has a thickness greater than about 0.014 inches and a thermalconductivity of about 0.1 w/(m.k.) or less faces the door of the testingapparatus, the Fire Endurance Index (FEI) of the board is greater thanabout 50 minutes.

In another aspect, a wall assembly comprises a first board comprising aset gypsum core disposed between first and second cover sheets, with thefirst and second cover sheets defining first and second board faces inopposing relation. The second cover sheet (e.g., back sheet) has athickness greater than about 0.014 inches, and a thermal conductivity ofabout 0.1 w/(m.k.) or less. The board has a dry weight of less thanabout 2000 lbs/1000 ft² when at a thickness of about ⅝inch. A secondboard comprises a set gypsum core, the set gypsum core defining thirdand fourth board faces in opposing relation, with the third and fourthboard faces each optionally in association with a cover sheet. The firstand second boards define an interior cavity of the wall assembly, suchthat the second cover sheet faces the interior cavity.

These and other advantages of the present invention, as well asadditional inventive features, will be apparent from the descriptionthat follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram displaying a cross section of a gypsum compositeboard in accordance with embodiments of the invention.

FIG. 2 is a diagram displaying a gypsum wallboard structure during afire test in accordance with embodiments of the invention.

FIG. 3 is a diagram displaying the structure of the small scale testdevice used to determine the Fire Endurance Index (FEI) of a wallboardsample in accordance with embodiments of the invention.

FIG. 4 is a line graph displaying the temperature profile (Y-axis) overtime (X-axis) of a furnace used during the small scale fire testillustrated in FIG. 3, in accordance with embodiments of the invention.

FIG. 5 is a line graph displaying a correlation between fire enduranceof U419 test (Y-axis) and fire endurance of small-scale test of FIG. 3(X-axis) in accordance with embodiments of the invention.

FIG. 6 is a line graph displaying surface temperatures (Y-axis) ofexposed wallboard inside a cavity over time (X-axis) for wallboards ofExample 1 during a standard fire test in accordance with embodiments ofthe invention.

FIG. 7 is a line graph displaying temperature gradient (Y-axis) across acavity over time (X-axis) for wallboards of Example 1 during a standardfire test in accordance with embodiments of the invention.

FIG. 8 is a line graph displaying the temperature of wallboard (Y-axis)over time (X-axis) for wallboards of Example 2 during a small-scale firetest in accordance with embodiments of the invention.

FIG. 9 is a line graph displaying the temperature of wallboard (Y-axis)over time (X-axis) for wallboards of Example 3 during a small-scale firetest in accordance with embodiments of the invention.

FIG. 10 is a line graph displaying the unexposed surface temperature(Y-axis) over time (X-axis) for wallboards of Example 4 during astandard fire test in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are premised, at least in part, onthe surprising and unexpected discovery that adding a thicker sheet to agypsum core yields a wallboard with greater fire endurance. While coversheets can act as an insulator, adding additional combustible materialto a wallboard would be expected to yield a board of lower fireendurance. Surprisingly, it was discovered that a wallboard comprising asheet of increased thickness and low thermal conductivity can increasethe fire endurance of a wallboard. In general, the composite gypsumboard of the present invention comprises a set gypsum core (e.g.,comprising an interlocking matrix of set gypsum) disposed between afirst cover sheet, which can be a face sheet, and a second cover sheet,which can be a back sheet. The second cover sheet (e.g., back coversheet) has a thickness greater than about 0.014 inches, and a thermalconductivity of about 0.1 w/(m.k.) or less. It has been discovered thatthe additional material of low thermal conductivity, when facing theinterior of a wallboard assembly, effectively lowers the heat transferrate through the wallboard, into the cavity and, ultimately, through thesecond wallboard. This technique is an alternative to known fireretardant methods using, e.g., corrosive fire retardant materials, andaffords a practical, cost-effective approach to increasing wallboardfire endurance.

In some embodiments, the gypsum board is part of an assembly thatcomprises two composite boards connected by studs 110 (FIG. 2). Thecomposite board itself can comprise a core 112, face sheet 114, and backsheet 116. The back sheet can be placed inside the cavity 118, such thatduring a fire outside the wall assembly, the back sheet faces away fromthe fire. In other words, the flames must pass through the face sheet114 and the core 112 before reaching the back sheet. The back sheethelps increase the thermal resistance of the wallboard. The additionallayer of back sheet can reduce the surface temperature on the back sideof the exposed wallboard, which lengthens the fire endurance of thewallboard and can limit potential structural damage.

In general, when a gypsum wallboard is under thermal stress, thermalenergy is initially directed to the evaporation of the calciumsulfate-bound water molecules. It is those two molecules of water thatrender gypsum highly resistant against heat. Upon reaching 215° F. watermolecules are driven off, which leads to the formation of calciumsulfate hemihydrates (CaSO₄.½H₂O). When the temperature reaches 250° F.,the remaining water is lost as gypsum is converted into calcium sulfateanhydrite. Both reactions are endothermic, meaning gypsum will absorbheat as it is “calcined” from dihydrate to anhydrite.

As gypsum is calcined, water molecules are lost and, unavoidably, thevolume occupied by the gypsum is reduced. The amount of volumetricshrinkage can range about 5-10% and can be affected by impurities in thegypsum. The shrinkage of gypsum board can affect the fire endurance ofgypsum boards. As wallboard shrinks, cracks may develop. These cracksallow heat to transfer through convection, accelerating the temperaturerise through the wallboard. In some cases, cracking of unexposed wallmay permit flame passage. Steel studs can exacerbate cracking as thedifferential expansion of metal studs can make a wall assembly deflecttowards the fire side. As a result, the exposed wallboard is undertension, aiding the development of cracks. Various core additives canhelp avoid crack formation. These additives include chopped fiberglass,vermiculite, and siloxane. The presence of chopped fiberglass helps touniformly distribute stress in the wallboard as a result of shrinkage,while the addition of vermiculite, which expands upon heating, may helpto prevent wallboard shrinkage. In general, such additives may helpmaintain wallboard structural integrity during a fire.

In a wallboard test assembly, heat is transferred from the furnace tothe surface of exposed board. As the surface temperature of the exposedwallboard increases, the temperature gradient across the boardincreases. If the wallboard maintains its structural integrity during afire, it effectively blocks the passage of flame and hot air. As aresult, the primary mode of heat transfer would be through conduction.The heat transfer rate during this stage would primarily depend upon thethermal conductivity of the wallboard. Inside the cavity formed betweentwo wallboards in a wall assembly, heat can be transferred from oneboard to the other by convection and conduction. Convection occurs whenair circulates inside the cavity. If insulation materials are placedinside the cavity, heat transfer is retarded. When metal studs are usedin a wall assembly, a significant amount of heat can be transferredthrough the studs as its thermal conductivity is relatively high. Whenwood studs are used, such as in test assembly U305, heat transfer insidethe cavity can be slowed.

Without wishing to be bound by theory, it is believed that the lowthermal conductivity and hydrophilic characteristics of the second sheet(e.g., back sheet) of the present invention are responsible for theobserved increase in fire endurance. When a fire occurs, the exposedsheet of the exposed wallboard can quickly burn off. However, the secondsheet (e.g., back sheet) when facing the interior of the cavity of awallboard assembly can remain on the gypsum core for an extended periodbecause the gypsum core, which comprises crystallized water molecules,provides protection against heat. The presence of a low thermalconductivity sheet helps increase the thermal resistance of the exposedwallboard against heat transfer. The core and second sheet can jointlyslow down heat transfer through the wallboard. Surprisingly, it has beenfound that as the thickness of the second sheet increases, the thermalresistance provided by the board increases, leading to improved fireendurance. In addition, the water released from calcination of thegypsum core can be re-adsorbed onto the hydrophilic sheet as waterpasses through the board. More thermal energy would be required to driveoff the water, which can slow heat transfer further. In this manner,water can be conserved in the wallboard during a fire to extend its fireendurance. For the wallboard on the ambient side, i.e., the side of thewall assembly that is away from the fire, similar mechanisms may helpslow down heat transfer. As the temperature increases, the second sheet(e.g., back sheet) of the board exposed to fire will eventually burnoff, but only after it has extended the fire endurance of the wallboardassembly.

To prepare a board, a mixture of at least stucco and water in slurryform can be deposited between two sheets to form a board preform. Atleast one sheet, which can be the back sheet, can comprise cellulosicmaterial, and has a low thermal conductivity. In general, at least onesheet can have a thickness greater than about 0.014 inches and a thermalconductivity of about 0.1 w/(m.k.) or less. In some embodiments, theboard has a dry basis weight of 2000 lbs/MSF when at a thickness of %inch. In some embodiments, the cover sheets are bonded to the set gypsumcore by a top and bottom high density bonding layer. When the core issufficiently hardened, it can be cut into one or more desired sizes toform individual gypsum boards. The boards can be transferred into andpassed through a kiln at temperatures sufficient to dry the panels to adesired moisture level. In some embodiments, an additional sheet can beadded by lamination onto the back surface of the second sheet (e.g.,back sheet), for example, using any suitable adhesive, such as 3M™ Super77™ adhesive. These thicker sheets can be used alone or in conjunctionwith other fire endurance additives and methods when manufacturingwallboards.

In another embodiment, a composite gypsum board can be made by forming amixture of at least stucco and water to make stucco slurry, depositingthe gypsum stucco slurry on a back sheet and forming a flat, relativelyuniform layer to create a board perform, wherein at least one sheet(e.g., back cover sheet) has a thickness greater than about 0.014inches, a thermal conductivity lower than about 0.1 w/(m.k.), andwherein the sheet is disposed on a back side of the board facing awayfrom the fire side when used in a board assembly, cutting the continuousboard preform into a board of predetermined dimensions after the slurryhas hardened sufficiently for cutting, and drying the board.

A wallboard of any thickness can be produced using the presentlydescribed methods and systems. The typical thickness of gypsum boards is½ inch and ⅝inch, but may range from ¼ inch to 1 inch. The totalwallboard thickness is defined, e.g., as the combined thickness of thefirst sheet (if present), second sheet (e.g., back sheet), and thegypsum core. In embodiments of the invention, the wallboard thicknesscan be, e.g., as listed in Table 1A. In the table, an “X” represents therange “from about [corresponding value in top row] to about[corresponding value in left-most column].” The indicated valuesrepresent the thickness of a board in inches (Table 1A). For ease ofpresentation, it will be understood that each value represents “about”that value. For example, the first “X” in Table 1A is the range “fromabout 0.59 inches to about 0.6 inches.” The ranges of the table arebetween and including the starting and endpoints.

TABLE 1A Starting Point for Wallboard Thickness (inches) 0.59 0.6 0.610.62 0.63 0.64 End Point for Wallboard 0.6 X Thickness (inches) 0.61 X X0.62 X X X 0.63 X X X X 0.64 X X X X X 0.65 X X X X X X

The cover sheets may comprise cellulosic fibers, glass fibers, ceramicfibers, mineral wool, or a combination of the aforementioned materials.The second sheet may comprise individual sheets or multiple sheets. Thesecond sheet has a total thickness of at least about 0.014 inches. Insome embodiments, it is preferred that there is a higher thickness,e.g., at least about 0.015 inches, at least about 0.016 inches, at leastabout 0.017 inches, at least about 0.018 inches, at least about 0.019inches, at least about 0.020 inches, at least about 0.021 inches, atleast about 0.022 inches, or at least about 0.023 inches. Morepreferably, the total second sheet thickness ranges from about 0.017inches to about 0.023 inches. The total second sheet thickness refers tothe sum of the thickness of each sheet attached to the gypsum board.

In embodiments of the invention, the thickness of the sheet can be,e.g., as listed in Table 1B below. In the table, an “X” represents therange “from about [corresponding value in top row] to about[corresponding value in left-most column].” The indicated valuesrepresent sheet thickness in inches. For ease of presentation, it willbe understood that each value represents “about” that value. Forexample, the first “X” in Table 1B is the range “from about 0.014 inchesto about 0.015 inches.” The ranges of the tables are between andincluding the aforementioned starting and endpoints.

TABLE 1B Starting Point for Sheet Thickness (inches) 0.014 0.015 0.0160.017 0.018 0.019 0.02 0.021 0.022 0.023 End Point for Sheet 0.015 XThickness (inches) 0.016 X X 0.017 X X X 0.018 X X X X 0.019 X X X X X0.02 X X X X X X 0.021 X X X X X X X 0.022 X X X X X X X X 0.023 X X X XX X X X X 0.024 X X X X X X X X X X

The thermal conductivity of the second sheet is less than about 0.1w/(m.k.). More preferably, the thermal conductivity of the second sheetis less than about 0.05 w/(m.k.). In embodiments of the invention, thethermal conductivity of the sheet can be, e.g., as listed in Table 1Cbelow. In the table, an “X” represents the range “from about[corresponding value in top row] to about [corresponding value inleft-most column].” The indicated values represent thermal conductivityin w/(m.k.). For ease of presentation, it will be understood that eachvalue represents “about” that value. For example, the first “X” in Table1C is the range “from about 0 w/(m.k.) to about 0.01 w/(m.k.).” Theranges of the tables are between and including the aforementionedstarting and endpoints.

TABLE 1C Starting Point for Thermal Conductivity Range (w/(m.k.)) 0 0.010.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 End Point for Thermal 0.01 XConductivity Range 0.02 X X (w/(m.k.)) 0.03 X X X 0.04 X X X X 0.05 X XX X X 0.06 X X X X X X 0.07 X X X X X X X 0.08 X X X X X X X X 0.09 X XX X X X X X X 0.1 X X X X X X X X X X

In preferred embodiments, the cover sheets comprises a cellulosic fiber.For example, paper sheet, such as Manila paper or kraft paper, can beused as the back sheet. Useful cover sheet paper includes Manila 7-plyand News-Line 5-ply, available from United States Gypsum Corporation,Chicago, Ill.; Grey-Back 3-ply and Manila Ivory 3-ply, available fromCaraustar, Newport, Ind.; and Manila heavy paper and MH Manila HT (hightensile) paper, available from United States Gypsum Corporation,Chicago, Ill. An exemplary cover sheet paper is 5-ply NewsLine. Inaddition, the cellulosic paper can comprise any other material orcombination of materials. For example, the second sheet can compriseglass fibers, ceramic fibers, mineral wool, or a combination of theaforementioned materials. The second sheet of the present invention isgenerally hydrophilic, meaning that the sheet is at least partiallycapable of adsorbing water molecules onto the surface of the sheetand/or absorbing water molecules into the sheet.

In other embodiments, the cover sheets be “substantially free” of glassfibers ceramic fibers, mineral wool, or a mixture thereof, which meansthat the cover sheets contain either (i) 0 wt. % based on the weight ofthe sheet, or no such glass fibers ceramic fibers, mineral wool, or amixture thereof, or (ii) an ineffective or (iii) an immaterial amount ofglass fibers ceramic fibers, mineral wool, or a mixture thereof. Anexample of an ineffective amount is an amount below the threshold amountto achieve the intended purpose of using glass fibers ceramic fibers,mineral wool, or a mixture thereof, as one of ordinary skill in the artwill appreciate. An immaterial amount may be, e.g., below about 5 wt. %,such as below about 2 wt. %, below about 1 wt. %, below about 0.5 wt. %,below about 0.2 wt. %, below about 0.1 wt. %, or below about 0.01 wt. %based on the weight stucco as one of ordinary skill in the art willappreciate. However, if desired in alternative embodiments, suchingredients can be included in the cover sheets.

Aluminum trihydrate, also known as alumina trihydrate and hydratedalumina, can increase fire resistance of gypsum wallboard due to itscrystallized or compound water content. In some embodiments, ATH can beadded in an amount from about 5% to about 30% by total weight of thesheet. ATH typically is very stable at room temperature. Abovetemperatures between about 180° C. and 205° C., ATH typically undergoesan endothermic decomposition releasing water vapor. The heat ofdecomposition for such ATH additives is greater than about 1000Joule/gram, and in one embodiment is about 1170 Joule/gram. Withoutbeing bound by theory, it is believed that the ATH additive decomposesto release approximately 35% of the water of crystallization as watervapor when heated above 205° C. in accordance with the followingequation: Al(OH)₃→Al₂O₃+3H₂O.

A cover sheet comprising inorganic particles of high water content, suchas ATH, can increase fire endurance of a gypsum wallboard. The inorganiccompound or mixture of compounds is incorporated into the sheet duringmanufacturing. Paper comprising ATH can be prepared by first dilutingcellulosic fiber in water at about 1% consistency, then mixing with ATHparticles at a predetermined ratio. The mixture can be poured into amold, the bottom of which can have a wire mesh to drain off water. Afterdraining, fiber and ATH particles are retained on the wire. The wetsheet can be transferred to a blotter paper and dried at about 200-360°F.

In one embodiment, the cover sheets can comprise any suitable amount ofinorganic compound or mixture of inorganic compounds that adequatelyimparts greater fire endurance. The cover sheets can comprise anyinorganic compound or mixture of inorganic compounds with highcrystallized water content, or any compound that releases water uponheating. In a preferred embodiment, the amount of inorganic compound orthe total mixture of inorganic compounds in the sheet ranges from about0.1% to about 30% by weight of the sheet. The inorganic compound orinorganic compounds used in the sheet may be of any suitable particlesize or suitable particle size distribution. In embodiments of theinvention, the percentage of inorganic compound or mixture of inorganiccompounds (e.g. ATH) by the total weight of the sheet can be, e.g., aslisted in Table 1D below. In the table, an “X” represents the range“from about [corresponding value in top row] to about [correspondingvalue in left-most column].” The indicated values represent thepercentage of inorganic compound or mixture of inorganic compounds bythe total weight of the sheet. For ease of presentation, it will beunderstood that each value represents “about” that value. For example,the first “X” in Table 1D is the range “from about 0 wt % to about 0.1wt %.” The ranges of the tables are between and including theaforementioned starting and endpoints.

TABLE 1D Starting Point for Inorganic Compound(s) Range (%) 0 0.1 0.5 12 5 10 15 20 25 End Point for 0.1 X Inorganic Com- 0.5 X X pound(s) (%)1 X X X Range 2 X X X X 5 X X X X X 10 X X X X X X 15 X X X X X X X 20 XX X X X X X X 25 X X X X X X X X X 30 X X X X X X X X X X

In some embodiments, e.g., ATH particles of less than about 20 μm arepreferred, but any suitable source or grade of ATH can be used. Forexample, ATH can be obtained from commercial suppliers such as Huberunder the brand names SB 432 (10 μm) or Hydral® 710 (1 μm).

In other embodiments, the cover sheet may comprise magnesium hydroxide.In these embodiments, the magnesium hydroxide additive preferably has aheat of decomposition greater than about 1000 Joule/gram, such as about1350 Joule/gram, at or above 180° C. to 205° C. In such embodiments, anysuitable magnesium hydroxide can be used, such as that commerciallyavailable from suppliers, including Akrochem Corp. of Akron, Ohio.

In other embodiments, the cover sheets be “substantially free” ofinorganic compounds such as ATH, magnesium hydroxide, or a mixturethereof, which means that the cover sheets contain either (i) 0 wt. %based on the weight of the sheet, or no such inorganic compounds such asATH, magnesium hydroxide, or a mixture thereof, or (ii) an ineffectiveor (iii) an immaterial amount of inorganic compounds such as ATH,magnesium hydroxide, or a mixture thereof. An example of an ineffectiveamount is an amount below the threshold amount to achieve the intendedpurpose of using inorganic compounds such as ATH, magnesium hydroxide,or a mixture thereof, as one of ordinary skill in the art willappreciate. An immaterial amount may be, e.g., below about 5 wt. %, suchas below about 2 wt. %, below about 1 wt. %, below about 0.5 wt. %,below about 0.2 wt. %, below about 0.1 wt. %, or below about 0.01 wt. %based on the weight of stucco as one of ordinary skill in the art willappreciate. However, if desired in alternative embodiments, suchingredients can be included in the cover sheets.

In embodiments where the core is sandwiched between two sheets, thecover sheets may comprise identical materials or materials withdifferent properties. For embodiments in which multiple second sheetsare used, the multiple sheets may comprise identical materials ormaterials with different properties.

The present invention can be practiced employing compositions andmethods similar to those employed in the art to prepare various setgypsum-containing products. In the core, the stucco (or calcined gypsum)component used to form the crystalline matrix typically comprises,consists essentially of, or consists of beta calcium sulfatehemihydrate, water-soluble calcium sulfate anhydrite, alpha calciumsulfate hemihydrate, or mixtures of any or all of these, from natural orsynthetic sources. In some embodiments, the stucco may includenon-gypsum minerals, such as minor amounts of clays or other componentsthat are associated with the gypsum source or are added during thecalcination, processing and/or delivery.

The gypsum core may comprise conventional additives in the practice ofthe invention in customary amounts to impart desirable properties and tofacilitate manufacturing, such as, for example, suitable starches,aqueous foam, leveling or nonleveling agents, set accelerators, setretarders, recalcination inhibitors, binders, adhesives, dispersingaids, thickeners, bactericides, fungicides, pH adjusters, colorants,reinforcing materials, fire retardants, water repellants, fillers,dimensional strengtheners, and mixtures thereof. In addition, the gypsumcore can comprise additives such as phosphonic and/or phosphonatecompounds, phosphoric and/or phosphate compounds, carboxylic and/orcarboxylate compounds, boric and/or borate compounds, and mixturesthereof.

The current trend in the industry is towards gypsum wallboard of alighter weight and lower density. A low basis weight can be achieved bymixing stucco slurry with a predetermined amount of foam based upontarget basis weight where the second sheet provides the fire endurancedisclosed therein. As the board contains less gypsum per unit volume,there is less crystallized water available for fire endurance of thewallboard. In addition, the percent shrinkage can increase as the boarddensity decreases. Both factors make it increasingly difficult to pass afire test. The present invention can provide high fire endurance forlightweight gypsum board. In preferred embodiments, the board, at athickness of about % inch, has a basis weight of less than about 2000lbs/1000 ft². In other preferred embodiments, the board, at a thicknessof about % inch, has a basis weight of less than about 1750 lbs/1000ft². The wallboard of the present invention may be of any basis weightwhere the second sheet provides the fire endurance as disclosed herein.In lightweight embodiments in accordance with the present invention, afoaming agent is employed to yield voids in the set known to be usefulin preparing foamed set gypsum products. Many such foaming agents arewell known and readily available commercially, e.g., from GEO SpecialtyChemicals in Ambler, Pa. For further descriptions of useful foamingagents, see, for example: U.S. Pat. Nos. 4,676,835, 5,158,612,5,240,639, and 5,643,510, which are, with regard to foaming agents,hereby incorporated by reference.

In some embodiments, the stucco slurry comprises about 2-4% vermiculite,about 0.5-5.0% starch, about 0.2-0.4% chopped fiberglass strands, about0.2-3.0% ground gypsum powder, about 0.2-2.0% phosphate compounds, andabout 0.01-0.1% dispersant. The starch can be of any type. For example,the starch can be pregelatinized, gelatinized in situ in the slurry, oracid-modified.

In some embodiments, assemblies can be constructed, using gypsum boardsformed according to principles of the present invention, that conform tothe specification of Underwriters Laboratories, Inc. (UL®) assemblies,such as U419, U305, and U423. The face of one side of the assembly canbe exposed to increasing temperatures for a period of time in accordancewith a heating curve, such as those discussed in the ASTM E119procedures (e.g., ASTM E119-09a). The temperatures proximate the heatedside and the temperatures at the surface of the unheated side of theassembly are monitored during the tests to evaluate the temperaturesexperienced by the exposed gypsum panels and the heat transmittedthrough the assembly to the unexposed panels. One useful indicator ofthe fire performance of gypsum panels in assemblies, for example thoseutilizing loaded, wood stud frames as called for in the ASTM E119 firetests, is discussed in the article Shipp, P. H., and Yu, Q.,“Thermophysical Characterization of Type X Special Fire Resistant GypsumBoard,” Proceedings of the Fire and Materials 2011 Conference, SanFrancisco, 31 Jan.-2 Feb. 2011, Interscience Communications Ltd.,London, UK, pp. 417-426. The article discusses an extensive series ofE119 fire tests of load bearing wood framed wall assemblies and theirexpected performance under the E119 fire test procedures. U.S. Pat. No.8,323,785 is incorporated by reference herein with regard to ASTM E119.

In some embodiments, an assembly of gypsum boards formed according toprinciples of the present invention and in accordance with thespecification of a U419 assembly, with or without cavity insulation, hasa fire rating of at least about 60 minutes when heated in accordancewith the time-temperature curve of ASTM standard E119-09. In someembodiments, an assembly of gypsum boards formed according to principlesof the present invention and in accordance with the specification of aU305 assembly has a fire rating of at least about 55 minutes when heatedin accordance with the time-temperature curve of ASTM standard E119-09.In some embodiments, an assembly of gypsum boards formed according toprinciples of the present invention and in accordance with thespecification of a U305 assembly has a fire rating of at least about 60minutes when heated in accordance with the time-temperature curve ofASTM standard E119-09. In some embodiments, an assembly of gypsum boardsformed according to principles of the present invention and inaccordance with the specification of a U423 assembly has a fire ratingof at least about 60 minutes when heated in accordance with thetime-temperature curve of ASTM standard E119-09.

In addition to common testing methods, the utility of the presentinvention to increase fire endurance can be analyzed using a small-scalefire endurance index (FEI) test. The FEI test is a small scale testingapparatus and method developed as an alternative to typical large scalewallboard testing. Fire endurance ratings are typically obtained byperforming a full-size (at 100 ft² of wall area) fire test in acertified fire test laboratory per ASTM standards, which istime-consuming, expensive, and unsuitable for bench-top studies andquality control.

A schematic diagram of a testing system 200 is shown, in cross section,in FIG. 3. The testing system 200 includes a muffle furnace 202 havingan enclosure 204 forming a furnace chamber 206. The chamber 206 iscloseable with a door 208 and includes a heat source 210 therewithin.The heat source 210 may be any known type of heat source such as afuel-fired combustor or an electric-resistive heater, which operates tocreate a generally uniformly distributed temperature profile within thechamber 206.

In the illustration of FIG. 3, a board sample 212 is shown disposedwithin the furnace chamber 206 during a test. The sample 212 is mountedvertically within the chamber 206 in the illustrated embodiment at anoffset distance from a door opening such that a gap 214 is formedbetween a back face 215 of the sample 212 and an oven-facing side of thedoor 208. Spacers 216 are disposed at a distance from one anotherbetween the sample 212 and the door 208 to simulate studs that spaceapart wallboards in a finished wall assembly. Although the gap 214 isshown empty, in an alternative embodiment the gap 214 may be filled witha wall-insulation material. Moreover, metal or wooden studs may be usedin place of the spacers 216. The spacers may be connected to the sample212 and, in certain embodiments, may be subjected to a compressive loadalong with the sample 212 to simulate a load-bearing wall.

A thermocouple 218 or other temperature-sensing device is connectedclose to the back face 215 of the sample during testing. The back face215 is thicker than the front face of the sample. The thermocouple 218has a sensing tip at a small distance from the surface of the sample212. In alternative embodiments, the sending tip can touch or be withinthe sample 212. The thermocouple 218 is configured to sense a surfacetemperature or a temperature near the surface of the back face of thesample 212 during testing. The thermocouple 218 is connected to a dataacquisition unit 220, which operates to provide power to thethermocouple 218, receive information therefrom indicative of thesurface temperature of the sample 212, record the temperatureinformation and, optionally or with the aid of a computer (not shown),plot the temperature information over time or otherwise analyze theinformation numerically.

When a test is conducted, the temperature of the muffle furnace chamber206 is gradually increased over time by appropriately controlling theintensity of the heat source 210. In one embodiment, a furnacetemperature sensor 222 is disposed to measure the temperature of thefurnace chamber 206, provide information indicative of the furnacechamber temperature to a heater controller 224 and, optionally, also tothe data acquisition unit 220. The heater controller 224 may operate ina closed loop fashion based on the information provided by the sensor222 to provide a predetermined heating profile for the chamber 206 byappropriately and automatically adjusting the intensity of the heatsource 210. The temperature rise of the chamber 206 may also optionallybe recorded by the data acquisition unit 220 for establishing testingintegrity.

A sample heating profile of the furnace chamber is shown in the timeplot of FIG. 4. As can be seen from the plot, where a desired chambertemperature (deg. F) is plotted along the vertical axis and time (min.)is plotted along the horizontal axis, the chamber 206 is heatedgradually following a logarithmic trend for about the first 43 minutesof the test from a temperature of about 400° F. to a temperature ofabout 1,423° F., and is maintained at that temperature for the remainderof the test, which in the illustrated graph continues for about 1 hour.Thus, the test is conducted over a first heating period, and thencontinues over a stable period, as marked on the graph of FIG. 4.

It has been determined that heat transfer through the sample 212 duringa test, as gleaned by the measured surface temperature on the back face215 of the sample, is concomitant to and indicative of the expected heattransfer through a wallboard in a full scale fire test. In essence, thetest describes herein determines the rate of heat transfer through thesample. In one embodiment, temperature readings taken on both sides ofthe board can be used to estimate, in real time, the heat transfer ratethrough the board. By comparing the heat transfer curves of differentproducts and correlating the curves to their actual fire test results,judgment and prediction of the performance of fire endurance ofdifferent products are advantageously enabled. In the test setup shownin FIG. 3, sample dimension was selected to be a rectangular samplehaving dimensions of 6.125″×6.625″ and a thickness of 0.625″. The depthof the cavity 214 was ⅞″, and the thermocouple 218 was located in thegeometrical center of the door 208, where the sensing probe of thethermocouple 218 protruded about 11/16″ from the inside surface of thedoor 208 in the direction of the sample 212. In this way, the tip of thethermocouple was 3/16″ away from the surface of the sample. A glass woolframe was placed against the sample to act as the spacer 216 and keepthe sample in place while also sealing the door frame against heatleakage. For half-inch thick samples, a metal frame of 0.125″ thicknesscan be placed behind the sample to maintain the gap between thethermocouple and the sample and preserve the remaining test setup. Thecontroller 224 of the muffle furnace was set to run from 200° C. to 773°C. The actual temperature curve of the muffle furnace at the front endis shown in FIG. 4.

The test provides a temperature-time curve for a specific board sample.Fire Endurance Index (FEI) can be determined from the curve. FireEndurance Index is defined as the time required to reach 600° F. at thebackside of a test specimen in the small scale fire test. Data points A,B, C, and D are plotted, and the correlation between FEI and fireendurance time from U419 full-size fire test is shown in FIG. 5. Otherdesigns of fire test assembly such as U305 and U423 can be extrapolatedfrom FEI as well.

In some embodiments, the composite gypsum board of the present inventionhas a Fire Endurance Index (FEI) greater than 52 minutes and at least 3minutes greater than a board comprising a sheet with a thickness lessthan about 0.014 inches. In some embodiments, the composite gypsum boardof the present invention has a Fire Endurance Index (FEI) greater than52 minutes and at least 4 minutes greater than a board comprising asheet with a thickness less than about 0.014 inches.

It shall be noted that the preceding are merely examples of embodiments.Other exemplary embodiments are apparent from the entirety of thedescription herein. It will also be understood by one of ordinary skillin the art that each of these embodiments may be used in variouscombinations with the other embodiments provided herein. The followingexamples further illustrate the invention but, of course, should not beconstrued as in any way limiting its scope.

Example 1 Effect of Thicker Sheet on Heat Transfer Across the Cavity

This Example demonstrates the effect of the sheet on heat transferacross the cavity of a U419 test assembly having no insulation in thecavity. For these tests, the surface temperature of the surface of theexposed wallboard inside the cavity and the temperature gradient acrossthe cavity, which are indicative of heat transfer through the boardduring a test, were monitored and recorded. The temperature gradientacross the cavity is defined as the temperature difference between theback sheet surface of the exposed board and the back sheet surface ofthe non-exposed board. Accordingly, a control board (sample A) and aboard with a thicker sheet (sample B) were tested in a full scale testusing a wall assembly of U419, with the thicker sheet facing theinterior cavity. The boards and sheets were produced at a plant with theadditives listed in Table 2 and with a water-to-stucco ratio of 0.91.Samples A and B had cellulosic paper with a thickness of 0.011 incheswith a basis weight of 39 lbs/1000 ft². An additional layer ofcellulosic paper sheet having a thickness of 0.011 inches and a basisweight of 40 lbs/1000 ft² was applied to Sample B. Sample B had a totalsheet thickness of 0.022 inches and total sheet basis weight of 79lbs/1000 ft². The two boards used in the assembly were of the sameweight and composition.

TABLE 2 wt % based on Additive weight of stucco Glass Fiber (JohnsManville, Denver, CO) 0.27 Vermiculite 3.84 (Virginia Vermiculite LLC,Louisa, VA) Diloflo Dispersant 0.05 (Geo Specialty Chemicals, Cleveland,OH) Ground Gypsum Powder 1.03 Starch (Bunge Milling, St. Louis, MO) 1.60Phosphate 1.08 (ICL Performance Products LP, St. Louis, MO)

The samples were heated according to ASTM E119-09. The temperatureprofile of the surface of the wallboard exposed to fire inside thecavity of the test wall assembly, and the temperature gradient acrossthe cavity, were then plotted in the graphs shown in FIGS. 6 and 7. Thetemperature trace for Samples A and B are shown in FIG. 6, where time isplotted along the horizontal axis and the cavity temperature for eachsample is plotted along the vertical axis. In FIG. 6, a dashed-linecurve 120 represents the temperature trace for the control (Sample A),and a solid-line curve 122 represents the temperature trace for SampleB. The temperature gradient across both samples is shown in FIG. 7. InFIG. 7, a dashed-line curve 124 represents the temperature gradienttrace for the control (Sample A), and a solid-line curve 126 representsthe temperature gradient trace for the board comprising a thicker sheet(Sample B).

This Example demonstrates that the presence of an additional layer ofsheet effectively reduced the surface temperature during the test on theback side of the exposed wallboard facing the cavity, indicating aslower heat transfer rate. Because the surface temperature on the heatedside of the cavity is reduced, the temperature gradient across thecavity is also effectively reduced (FIG. 7). Consequently, reaching thefailure temperature on the surface of wallboard on the ambient side isdelayed, which increases the fire endurance of the wallboard assembly.

Example 2 Effect of Sheet Thickness on Fire Endurance

This Example demonstrates the effect of sheet thickness on the fireendurance of a wallboard. A control board (sample C) and a board with athicker sheet (sample D) were tested using the Fire Endurance Index(FEI) small-scale testing device described above relative to FIG. 3. Aboard (4 ft×10 ft) was produced at a plant with the additives listed inTable 3, a water-to-stucco ratio of 0.91, a thickness of 0.617 inches, abasis weight of 1681 lbs/1000 ft², and a density of 32.72 lbs/ft³. Theboard had cellulosic paper with a thickness of 0.012 inches and a basisweight of 42 lbs/1000 ft². The board was cut into samples of 6.625inches×6.125 inches. To make sample D, an additional layer of cellulosicsheet having a thickness of 0.011 inches and a basis weight of 39lbs/1000 ft² was laminated onto one of the cut boards using 3M™ Super77™ adhesive. After lamination, the sheet of sample D had a totalthickness of 0.023 inches and basis weight of 81 lbs/1000 ft². Theamount of adhesive applied was approximately 3 lbs/1000 ft². The sheetshad a thermal conductivity of 0.05 w/(m.k).

TABLE 3 wt % based on Additive weight of stucco Glass Fiber (JohnsManville, Denver, CO) 0.27 Diloflo Dispersant 0.05 (Geo SpecialtyChemicals, Cleveland, OH) Ground Gypsum Powder 1.03 Starch (BungeMilling, St. Louis, MO) 1.60 Phosphate 1.08 (ICL Performance ProductsLP, St. Louis, MO)

After the boards were allowed to condition at ambient conditions for atleast 24 hours, the boards were individually tested in the small-scaledevice (FIG. 3) to determine their respective FEI values. The coversheet was faced the door of the testing apparatus. Temperature tracesfor each of the two samples are shown in FIG. 8, where time is plottedalong the horizontal axis and temperature of the back-face of eachsample is plotted along the vertical axis. In the graph of FIG. 8, thesolid line 302 represents the temperature trace for the control (sampleC), the dashed line 304 represents the temperature trace for sample D,and a “FEI” line extends horizontally from a temperature of 315.6° C.(600° F.) to denote the FEI indexes for both samples.

As can be calculated from the graph of FIG. 8, the Fire Endurance Indexwas 48.0 minutes for the control sample (sample C) and 52.0 minutes forsample D. The increase in fire endurance time was 4.0 minutes when anadditional layer of cellulosic sheet of 0.011 inches thickness was addedto the wallboard.

Example 3 Fire Endurance of Sheets Comprising ATH

This Example demonstrates the effect of using a sheet comprisinginorganic additives on the fire endurance of a wallboard. In addition,this Example illustrates the impact of inorganic particle size on thefire endurance of a wallboard. Specifically, inorganic compound aluminumtrihydrate (ATH) was used as additive. Accordingly, a control board(sample E), a board comprising ATH with a particle size of about 10 μm(sample F), and a board comprising ATH with a particle size of about 1μm (sample G) were tested using the Fire Endurance Index (FEI)small-scale testing device (FIG. 3).

Three individual 1 ft×1 ft gypsum board samples were prepared using theadditives of Table 3, a water-to-stucco ratio of 1.9, and constructedusing a laboratory casting device. The control sample (sample E) wasmade with a cellulosic sheet of 0.0161 inches thickness and 42.1lbs/1000 ft² basis weight. The board had a thickness of 0.630 inches anda basis weight of 1632 lbs/1000 ft². Sample F was made using acellulosic sheet of 0.0196 inches thickness and 51.6 lbs/1000 ft² basisweight, and comprised 16.34% ATH with a particle size of about 10 μm.The board had a thickness of 0.640 inches and a basis weight of 1617lbs/1000 ft². ATH with a 10 μm particle size was purchased from Huberunder the brand name Hymod® SB 432. Sample G was produced using acellulosic sheet of 0.0184 inches thickness and 52.3 lbs/1000 ft² basisweight, and comprised 17.49% ATH with a particle size of about 1 μm. Theboard had a thickness of 0.638 inches and basis weight of 1631 lbs/1000ft². ATH with a 1 μm particle size was purchased from Huber under thebrand name Hydral® 710. The sheets described above for samples E, F, andG all had a thermal conductivity of 0.05 w/(m.k).

After casting, drying, and conditioning at ambient conditions for atleast 24 hours, the board samples were cut into samples of 6.125inches×6.625 inches. After the boards were allowed to condition atambient conditions for at least another 24 hours, the boards wereindividually tested in the small-scale device (FIG. 3) to determinetheir respective Fire Endurance Index (FEI) where the cover sheet facedthe door of the testing apparatus. Temperature traces for each of thethree samples are shown in FIG. 9, where time is plotted along thehorizontal axis and temperature of the back-face of each sample isplotted along the vertical axis. In the graph of FIG. 9, the long-dashedline 306 represents the temperature trace for the control (sample E),the solid line 308 represents the temperature trace for the boardcomprising a sheet with ATH (10 μm) (sample F), and the short-dashedline 310 represents the temperature trace for the board comprising asheet with ATH (1 μm) (sample G).

As can be calculated from the graph of FIG. 9, the Fire Endurance Indexwas 57 minutes and 15 seconds for the control (sample E), 60 minutes 30seconds for the sample comprising 16.34% ATH with a particle size ofabout 10 μm (sample F), and 61 minutes and 40 seconds for the samplecomprising 17.49% ATH with a particle size of about 1 μm (sample G). Incomparison with the control, the average increase in the Fire EnduranceIndex was about 3 minutes and 50 seconds. As demonstrated in FIG. 9, thetemperatures were consistently lower when a sheet comprising ATH wasused, indicating a slower heat transfer rate. In other words, the boardswith a sheet comprising ATH (samples F and G) had greater fire endurancethan the board comprising a sheet without ATH (sample E). Furthermore,the board comprising a sheet comprising ATH particles of about 1 μm(sample G) had a greater fire endurance than the board comprising asheet comprising ATH particles of about 10 μm (sample F). This Exampledemonstrates that a sheet comprising ATH particles can increase the fireendurance of a gypsum wallboard.

Example 4 Surface Temperatures of U419 Test Assembly

This Example illustrates the effect of a thicker sheet facing the cavityof a wallboard assembly on the fire endurance of a U419 assembly designhaving no insulation in the cavity, with the thicker sheet facing theinterior cavity. Accordingly, a control sample (sample H) and a samplewith a thicker sheet facing the cavity (sample I) were tested in astandard U419 fire test. The boards and sheets were produced at a plantwith the additives listed in Table 2, a water-to-stucco ratio of 0.91, aboard thickness of 0.620 inches, basis weight of 1723 lbs/1000 ft², anddensity of 33.35 lbs/ft³. Both boards were fitted with a cellulosicpaper of 0.011 inches thickness, and 40 lbs/1000 ft² basis weight. Tomake sample I, an additional layer of cellulosic sheet was laminatedonto one surface of the existing board using 3M™ Super 77™ adhesive.After lamination of sample I, the sheet had a total thickness of 0.022inches and basis weight of 79 lbs/1000 ft². The amount of adhesiveapplied was approximately 3 lbs/1000 ft². The sheets described above forsamples H and I had a thermal conductivity of 0.05 w/(m.k). The twoboards used in the assembly were of the same weight and composition.

A sample area of 100 ft² was heated according to ASTM E119-09.Temperature traces for each of the two samples are shown in FIG. 10,where time is plotted along the horizontal axis and temperature of thenon-exposed sheet of each sample is plotted along the vertical axis. InFIG. 10, a dashed-line curve 312 represents the temperature trace forthe control sample (sample H), and a solid-line curve 314 represents thetemperature trace for the board comprising the thicker sheet (sample I).Based on the conditions of the test, it was determined that the controlboard (curve 312) had a fire endurance of 49 minutes and 53 seconds,while the sample I (curve 314) had a fire endurance of 53 minutes and 16seconds. In other words, the treatment appeared to increase boardendurance by about 3 minutes and 23 seconds.

This Example further demonstrates that a thicker sheet as describedabove increases the fire endurance of a gypsum wallboard. In addition,these results for a full scale test corroborate what was observed in thesmall scale test of Example 2.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Also, everywhere “comprising”(or its equivalent) is recited, the “comprising” is considered toincorporate “consisting essentially of” and “consisting of” Thus, anembodiment “comprising” (an) element(s) supports embodiments “consistingessentially of” and “consisting of” the recited element(s). Everywhere“consisting essentially of” is recited is considered to incorporate“consisting of” Thus, an embodiment “consisting essentially of” (an)element(s) supports embodiments “consisting of” the recited element(s).Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1-23. (canceled)
 24. A composite gypsum board comprising: a corecomprising set gypsum formed from at least water and stucco, the corehaving a substantially flat shape and a generally uniform thickness, thecore defining first and second board faces in opposing relation; a sheetdisposed in bonding relation along the first board face, the sheet madeof a material that has a thickness greater than about 0.017 inches,possesses a thermal conductivity that is less than about 0.1 w/(m.k.),and is applied to a back side of the composite gypsum board; and thecomposite gypsum board having a dry weight of less than about 1750lbs/1000 ft² when at a thickness of about ⅝inch.